The invention generally relates to electrical measuring equipment and methods. In particular, the invention relates to microwave equipment and methods for mapping resistivity, thickness, and other electrical characteristics over a surface with resolution of a few micrometers, that is, a microwave microscope.
In U.S. Patent 5,781,018, two of us, Davidov and Golosovky, describe a microwave microscope including a single resonant slit. We describe an improvement of this microwave microscope in U.S. Pat. No. 6,100,703 including a crossed pair of resonant slits. Both these patents are incorporated by reference in their entireties.
The prior-art microscope of U.S. Pat. No. 5,781,018, as illustrated schematically in FIG. 1, includes a microwave probe 12 formed of a rectangular waveguide 14, the probe end of which is covered with a thin conductive foil 16. A resonant slit 18, to be described in more detail later, is formed in the conductive foil 16. The probe end is positioned over a front surface 20 of a sample 22 to be scanned. For example, the sample 22 may be a silicon wafer covered by metallic and/or dielectric layers and having various very small features formed in its surface 20 which need to be electrically characterized. The sample 22 is mounted on an X-Y stage 26 driven by a drive 28 under the control of a computer 30 so as to allow the probe 12 to be scanned over the sample surface 20. A source 32 of microwave radiation, provides the probing microwaves to a microwave bridge formed of a hybrid tee, an adjustable attenuator 36, a sliding short 38, and an E-H tuner 40 that matches the impedance of the probe antenna (slit) 18 to that of the waveguide 14. A microwave detector 42 receives radiation from the bridge to thereby measure its imbalance, and the intensity is transmitted to the computer 30. The amount of imbalance is determined, in part, by the electrical characteristics of the material in the sample 22 immediately below the slit 18, and thus can be used to measure those electrical characteristics on a scale equal to the dimensions of the slit. In the patent, the microwave radiation is in the millimeter band, about 80 GHz.
In a preferred embodiment illustrated orthographically in FIG. 2, the probe 12 is formed with a one-dimensionally curved foil end 16. The long dimension of the slit 18 extends along the curve, and the short dimension of the slit 18 is transverse to the curve. As is described in detail in the two patents, if the long dimension of the slit 18 is made nearly resonant with the probing radiation, that is, half of a free-space wavelength (a few millimeters at 80 GHz), the slit""s short dimension can be made nearly arbitrarily small, but the probe end remains nonetheless nearly transparent to the microwave radiation. If the slit""s short dimension is formed to the order of micrometers or somewhat less, the short dimension defines the sampling resolution of the probe along the transverse direction of the slit.
The mechanical stability of the convexly curved probe end is improved, as illustrated in the side cross-sectional view of FIG. 3, by placing a low-loss dielectric body 44 at the end of the probe. Its curved front end supports the thin foil 16. Its sides fit closely to the rectangularly shaped waveguide 14. Its pyramidally shaped back end minimizes microwave reflection. To further minimize microwave reflection, the dielectric body should have a low dielectric constant, for example, xcex5=2.2. U.S. Pat. No. 5,781,018 also teaches dielectrically loading the entire microwave waveguide.
The U.S. Pat. No. 6,100,703 includes two resonant slits arranged perpendicularly. Polarization-sensitive detection equipment then allows separated detection of the incident polarization and the perpendicular polarization. The rotated polarization (90xc2x0 rotated) is particularly useful for mapping Hall mobilities, anisotropies, and local non-uniformities.
An important task for the near-field microwave microscope is the contactless characterization of conductive layers, particularly thickness mapping of thin metal layers overlying a dielectric underlayer. For conductive films that are thinner than the skin-depth of their constituent metal, the characterization may be conveniently performed through the local measurement of the sheet resistance Rsh=p/t, where p is the bulk resistivity of the metal and t is the thickness of the metal layer. The metal layers most used in semiconductor fabrication are composed of Cu, Ag, Al, and W and have thicknesses in the range of 0.1 to 3 xcexcm and sheet resistances in the range of 0.1 to 10 xcexa9. The skin-depths for these materials at microwave frequencies between 1 and 100 GHz are in the range of 0.2 to 1 xcexcm. Thus, a microwave microscope of proper design can in many cases characterize the sheet resistance and thus thickness of these metal layers. In the fabrication of semiconductor integrated circuits, it is often important to determine the uniformity of metal deposition to assure a sufficiently thick metal layer on all portions of the wafer.
However, the use of a microwave microscope for conductive layer of appreciable thickness, even of a significant fraction of a skin depth, requires the use of low impedance probe. The sensitivity of a simple slit probe and of most popular microwave probes is such that they allow the characterization of conductive films with sheet resistances of 100 xcexa9 or greater. This sensitivity is not enough to effectively probe metal layers of sheet resistance of less than 10 xcexa9. Hao et al. have disclosed a low-impedance scanning dielectric resonator in xe2x80x9cSpatially resolved measurements of HTS microwave surface impedance,xe2x80x9d IEEE Transaction in Applied Superconductivity, vol. 9, no. 2, Jun. 1999, pp. 1944-1947. However, their resolution of a few millimeters is not fine enough for characterizing the small features of modern semiconductor integrated circuits.
Accordingly, it is desired to provide a probe capable of electrically characterizing conductive layers on a semiconductor wafer. In particular, it is desired to provide such a probe for layers having sheet resistance of less than 10 ohms.
It is further desired to provide a microwave probe antenna that has a narrow slit and a low electrical impedance.
A microwave microscope including a narrow resonant slit in a conductive end of the probe tip and a dielectric resonator in the waveguide behind the resonant slit to impedance match the waveguide to the high impedance slit. The dielectric resonator is formed by a resonator member having a high dielectric constant, placed next to the resonant slit, and having a resonant length, of the order of the microwave radiation in the material. A long dielectric member is placed in back of the resonator member and separated from the resonator member by a small gap having a width chosen to form an impedance transformer matching the waveguide impedance to the impedance of the combination of the slit and resonator member. The gap width preferably is in the range of 0.1% to 100% of the free-space wavelength of the microwave radiation. The gap may be operationally set by varying its length to minimize microwave reflection from assembly of the slit and the resonator member.
The front end of the resonator member, that is, the end facing free space may be flat or preferably convex. The conductive end of the waveguide and the slit may be formed by coating this front surface of the resonator member with a metal layer and forming the slit in the coated metal.